Efficient and Selective Photogeneration of Stable N-Centered Radicals via Controllable Charge Carrier Imbalance in Cesium Lead Halide Nanocrystals

Despite the versatility of semiconductor nanocrystals (NCs) in photoinduced chemical processes, the generation of stable radicals has been more challenging due to reverse charge transfer or charge recombination even in the presence of sacrificial charge acceptors. Here, we show that cesium lead halide (CsPbX3) NCs can selectively photogenerate either aminium or aminyl radicals from amines, taking advantage of the controllable imbalance of the electron and hole populations achieved by varying the solvent composition. Using dihalomethane as the solvent, irreversible removal of the electrons from CsPbX3 NCs enabled by the photoinduced halide exchange between the NCs and the dihalomethane resulted in efficient oxidative generation of the aminium radical. In the absence of dihalomethane in solvent, the availability of both electrons and holes resulted in the production of an aminyl radical via sequential hole transfer and reductive N–H bond dissociation. The negative charge of the halide ions on the NC’s lattice surface appears to facilitate the aminyl radical production, competing favorably with the reversible charge transfer reverting to the reactant.


■ INTRODUCTION
−18 While semiconductor NCs can produce various species from the transfer of electrons or holes to the reactants, the generation of stable and persistent radicals via photoinduced charge transfer has been more challenging.Radicals are typically produced from closed-shell precursors via charge transfer and bond dissociation.Charge transfer from molecular redox photocatalysts such as metal complexes 19,20 or the electrochemical cell 21 is a common approach widely used in addition to the traditional chemical methods.When metal complex photocatalysts are used for radical production via charge transfer, back charge transfer that prohibits the effective formation of radicals is suppressed by modifying the ligand or by introducing irreversible downstream reactions. 22,23In principle, semiconductor NCs can also perform the photoinduced charge transfer necessary to convert the precursors to radicals reductively or oxidatively.However, the problems arising from back charge transfer or charge recombination are more difficult to address in semiconductor NCs using the same approaches.Although such problems can be partially mitigated by using sacrificial charge acceptors, 24 reversible charge transfer cannot be completely blocked, often resulting in the generation of radicals only transiently, even for the species known to be stable in the absence of reversible charge transfer.
Here, we report a perovskite NC-based method that effectively prevents the unproductive reversible charge transfer, enabling the efficient generation of an N-centered cation (aminium) or a neutral (aminyl) radical in a selective manner stably in solution and also performing radical−radical coupling reaction for a more reactive pair of radicals.The new method utilizes the unique ability of cesium lead halide (CsPbX 3 ) NCs to control the population imbalance of the electrons and holes transferring to the precursor to perform either fully oxidative or redox-neutral radical formation, as illustrated in Scheme 1.The control of the electron and hole population imbalance is achieved via the photoinduced halide exchange reaction between the CsPbX 3 NCs and the dihalomethane (CH 2 X 2 ) solvent, 25 where the exchanging X − comes from the reductive dissociation of CH 2 X 2 by the NCs.The halide exchange process is essential for irreversibly removing the electrons transferring from the NCs to CH 2 X 2 , which enables exclusive oxidative radical formation.When both electrons and holes are available from the NCs in the absence of CH 2 X 2 , a redox-neutral aminyl radical was formed stably via hole transfer, followed by reductive N−H bond breaking.The presence of the negative charge on the lattice surface, provided by halide ions, seems to assist the formation of aminyl radicals competing with the net null reaction resulting from the reversible charge transfer.This is an important advantage over other semiconductor NC photocatalysts such as II−VI quantum dots 26−28 that exhibit a more limited capability to produce a stable population of aminium and aminyl radicals as compared in this work.The coupling reactions between the photogenerated radicals are also demonstrated, where the reaction benefits from a controllable charge carrier imbalance.We anticipate that the capability of CsPbX 3 NCs demonstrated here can be expanded to other radical precursors, and the NC's function can be enhanced by varying the lattice and ligand structure.

■ RESULTS AND DISCUSSION
To investigate the capability of CsPbX 3 NCs to effectively produce and accumulate organic radicals, we chose several secondary aryl amines and tertiary amines as precursors that are known to produce radicals via chemical or electrochemical methods.In particular, we show that stable aminium and aminyl radicals can be selectively produced from the same secondary aryl amine precursor using different solvent compositions that create different electron and hole population imbalance conditions.For the photoexcitation with visible light that is not absorbed by solvent and amine precursors, all the reactions were carried out using CsPbBr 3 NCs, exhibiting exciton transition in visible wavelengths, although CsPbCl 3 NCs with a higher band gap can perform the same reactions.The absorption spectrum and a TEM image of CsPbBr 3 NCs are in the Supporting Information (Figure S1).Reducing the size of CsPbBr 3 NCs to impose quantum confinement may affect the kinetics of the charge transfer reaction by altering the band edge level and the available surface area on the NCs.However, the impact may not be as pronounced as in the photoreaction mediated by the Dexter-type energy transfer from CsPbX 3 NCs to triplet acceptors in a recent study. 29We chose CH 2 Br 2 for dihalomethane solvent since the self-halide exchange between CsPbBr 3 NCs and Br − allows us to avoid changing the band gap of the NCs that depends on the halide composition. 25The identities of all the reaction products were confirmed by using the combination of UV−visible absorption, electron paramagnetic resonance (EPR), and mass spectrometry (MS), as detailed in the Supporting Information.MS data provided particularly informative and detailed structural information on the radical species due to its high sensitivity and specificity for product ion analysis in a mixture.
Table 1 summarizes the structure of the precursors and radicals formed using CsPbBr 3 NCs as the photocatalyst under     1 is that two different radical species are formed under different solvent conditions from the same secondary aryl amine precursor, such as phenothiazine (PTZ) and phenoxazine (POZ).When a 1:1 v/v mixture of hexane and CH 2 Br 2 is used as the solvent, the aminium radical is formed exclusively via one-electron oxidation.Although CH 2 Br 2 does the same reaction, 50% diluted CH 2 Br 2 in hexane was used to improve the dispersion of the NCs in solvent.When only hexane is used as the solvent, the aminyl radical is formed instead of the aminium radical, which is the hydrogen atom abstraction product of the same precursor.In the CH 2 Br 2 /hexane solvent mixture, electrons in the photoexcited CsPbBr 3 NCs were removed irreversibly, resulting in the effective one-electron oxidation of amines via hole transfer undisturbed by the electrons at the conduction band (Scheme 1a).The combination of the reductive dissociation of CH 2 Br 2 and the photoinduced halide selfexchange is considered to create a kinetically favorable pathway that consumes electrons by forming the gaseous reduction products of CH 2 Br 2 over the reversible back electron transfer to NCs.Earlier studies reported the observation of methane and ethylene produced as the electrochemical reduction of CH 2 Br 2. 30,31 In contrast, when only hexane is used as the solvent, both electrons and holes are available for the oxidative and reductive charge transfer processes.The initial oxidation followed by the reductive deprotonation breaking the N−H bond is a plausible pathway to form an aminyl radical, 32,33 which is in competition with the sequential oxidation and reduction recovering the initial precursor (Scheme 1b).
To examine the capability of CsPbBr 3 NCs to produce the radicals stably under the reaction conditions listed in Table 1, the progress of the reaction was monitored by measuring the characteristic absorption of each radical species.The stability of the radicals was also examined by monitoring the dissipation of the absorption from the photogenerated radicals in the dark after the photoexcitation.In addition, ex situ EPR spectroscopy further confirmed the presence of the radical species that could coexist stably with NCs in the solution (Figure S2). Figure 1a,d shows the time-dependent absorption spectra of the reaction products from the mixture of PTZ and CsPbBr 3 NCs in two different solvents, i.e., the CH 2 Br 2 /hexane mixture and hexane, under 473 nm excitation.For clarity, the absorption from CsPbBr 3 NCs that did not change during the reaction was subtracted from all raw absorption spectra to show only the net changes in the absorption in Figure 1a,d.The raw absorption spectra at different reaction times are in the Supporting Information (Figure S3).CsPbBr 3 NCs were stable without exhibiting any sign of degradation during the reaction (>3 h), likely due to the efficient removal of both charge carriers.Since PTZ does not exhibit a measurable absorption at wavelengths longer than 360 nm, the spectra shown in Figure 1a,d reflect only the reaction product of PTZ, which match well with the reported absorption spectra of aminium (PTZ +• ) and aminyl (PTZ-H • ) radicals, respectively. 34,35igure 1b,e shows the time-dependent absorption intensities of the two radical species measured at 523 and 387 nm, respectively, corresponding to the peak of absorption from PTZ +• and PTZ-H • .The increasing absorption intensity with the reaction time in Figure 1b indicates that PTZ +• is continuously produced and accumulates in the solution.When the photoexcitation is discontinued, the absorption intensity of PTZ +• decreases by only ∼10% after 3 h, as shown in Figure 1c, demonstrating the stability of PTZ +• .The time-dependent absorption of PTZ-H • shown in Figure 1e exhibits a more rapid increase and saturation under the same photoexcitation condition.When the photoexcitation is discontinued, the absorption intensity of PTZ-H • decays faster than that of PTZ + , • diminishing by ∼65% after 3 h (Figure 1f) indicating that PTZ +• is more stable than PTZ-H • under the employed reaction condition.It is informative to compare the absorption spectra of PTZ +• and PTZ-H • with those obtained under several additional control reaction conditions.In Figure 1g, absorption spectra taken after 1 h of reaction under four additional reaction conditions are compared with the spectra of PTZ +• and PTZ-H • at 1 h of reaction time on the same absorbance scale.The details of the additional control reaction conditions are given in the caption of Figure 1g.One is using benzoquinone (BQ) as the electron acceptor in the hexane solution of CsPbBr 3 NCs (condition 1), which exhibits only a weak and indistinct absorption after 1 h of reaction.While BQ accepts the electron from CsPbBr 3 NCs, the electron can be transferred back from the reduced BQ (BQ −• ) to the valence band of the NCs removing the hole or to other electron acceptors that may exist in the solvent medium.A recent study reported ∼65 ps electron transfer time from CsPbBr 3 NCs to BQ and ∼50 ps hole transfer time from CsPbBr 3 NCs to PTZ at the concentrations of BQ and PTZ comparable to this study (5 mM). 36The same study also reported a nanosecond time scale for the charge recombination between CsPbBr 3 NCs and BQ −• or PTZ +• .The fact that neither PTZ +• nor PTZ-H • is observable in the absorption spectrum of condition 1 suggests that the back electron transfer from BQ −• to PTZ +• may be the dominant pathway prohibiting the production of any radical species, which contrasts to the reaction in Scheme 1a with an irreversible electron removal pathway.
For conditions 2−4, CsPbBr 3 NCs were replaced with CdSSe NCs having the same band gap and similar ligands as those of CsPbBr 3 NCs under three different solvent compositions, i.e., hexane, hexane with BQ as the electron acceptor, and the CH 2 Br 2 /hexane mixture.The absorbance of CdSSe NCs at the excitation wavelength (473 nm) was matched to that of CsPbBr 3 NCs to produce the same total number of photoexcited charge carriers by the NCs under all reaction conditions.Only in hexane solvent (condition 2), a weak absorption feature that can be attributed to PTZ-H • is observed analogously to the case of CsPbBr 3 NCs in hexane, albeit at a much lower intensity.The other two reaction conditions (condition 3 and 4) did not produce any significant product identifiable in the absorption spectra.CdS NCs with the valence bandedge level close to that of CsPbBr 3 NCs 37 were also ineffective in generating radicals in a separate measurement.The above comparison suggests that the ability to irreversibly remove electrons is the key to the difference between the two NCs in their ability to oxidatively generate aminium radicals, rather than the thermodynamic potential for charge transfer.In the earlier studies, the reduction of the dissolved O 2 in solvent was proposed to be capable of removing the photoexcited electrons from CsPbBr 3 NCs driving the oxidative reaction. 16,17However, in the present study, which generates stable radicals from amines, O 2 does not play a significant role in consuming the photoexcited electrons necessary to perform the oxidative aminium radical formation.In the absence of CH 2 Br 2 in the solvent, CsPbBr 3 NCs do not produce PTZ +• but produce PTZ-H • even in the presence of dissolved O 2 , while the presence of O 2 adds more complexity in the reaction (Figure S4).
The selective production of aminium and aminyl radicals by CsPbBr 3 NCs by introducing charge carrier population imbalance was also possible for POZ, another secondary aryl amine.Figure 2a,d shows the time-dependent absorption spectra of the reaction products from the mixture of POZ and CsPbBr 3 NCs in two different solvents under the same photoexcitation condition as in PTZ.The absorption peaks (380, 407, and 528 nm) in Figure 2a are characteristic of POZ +• . 38,39The peak (367 nm) in Figure 2d is attributed to POZ-H • . 39,40Figure 2b,e shows the time-dependent absorption intensities of the two radical species measured at 528 and 367 nm from POZ +• and POZ-H • , respectively.Figure 2c,f shows the decay of absorption from POZ +• and POZ-H • in the dark over 2 h.While the time scales of the buildup and decay of the absorption intensities are different from those of PTZ, both PTZ and POZ show the same selective formation of different radicals in different solvents.In Figure 2g, absorption spectra obtained under four additional reaction conditions are shown with the spectra of POZ +• and POZ-H • , similarly to the comparison made in Figure 1g.Adding BQ to the hexane solution of CsPbBr 3 NCs (condition 1) results in a spectrum similar to that of POZ-H • , suggesting that the reductive deprotonation of POZ +• by BQ −• is more facile than that in the case of PTZ +• .As in the case of PTZ, CdSSe NCs are not effective in generating radical species under all solvent conditions (condition 2−4).
Halide exchange in CsPbBr 3 NCs with Br − generated from the reductive dissociation of CH 2 Br 2 appears to be crucial for the irreversible removal of the electrons from CsPbBr 3 NCs necessary for the stable accumulation of the photogenerated aminium radical.Our recent study showed that the photoexcited II−VI NCs also produce Br − via the reductive dissociation of CH 2 Br 2 similar to perovskite NCs. 25 This was We hypothesize that the reductive N−H bond breaking of the intermediate species (PTZ +• ) requires interaction with the NC surface acting as an active site since a simple one-electron reduction of PTZ +• would recover the initial reactant (PTZ).We further hypothesize that the anion-terminated NC surface may provide a better environment for the positively charged PTZ +• to interact with the NC surface.Since CsPbBr 3 and CdS NCs prepared using typical synthesis methods have Br −rich and Cd 2+ -rich surfaces, respectively, for the higher photoluminescence intensity, we examined the effect of having the ions of the opposite charge on the surface of each NC sample.To this end, we compared the initial production rate of PTZ-H • by CsPbBr 3 and CdS NCs each with both cation-rich and anion-rich lattice surfaces, as shown in Figure 3.The details of the difference in the properties of NCs with the cation-rich and anion-rich lattice surface are in the Supporting Information (Figures S6 and S7).While the role of the ligand headgroup cannot be easily separated, we found that Br −terminated CsPbBr 3 NCs passivated with oleylammonium bromide (OLAB) are more effective in producing PTZ-H • than Na + -terminated CsPbBr 3 NCs without long surface ligands, despite the more steric hindrance on CsPbBr 3 NCs with the OLAB ligand.Similarly, CdS NCs produce PTZ-H • more effectively when the NC surface is anion-rich for similar surface ligands (oleylamine vs oleic acid).Our results indicate that PTZ-H • generation via multistep charge transfer and bond dissociation can be altered by varying the surface atoms of the NC, although further studies are needed to draw a more definitive conclusion.It is noteworthy that an earlier study observed the enhancement of diamine oxidation and cyclization by introducing Cu 2+ ions to CsPbBr 3 NCs that provided binding sites for the reactant. 17These suggest the possibility of further tuning the radical formation reactions by varying the surface ion composition of CsPbX 3 NCs as well as the ligand that has shown the ability to exhibit selective reaction such as in stereoselective radical coupling. 16or the tertiary amines, only the production of aminium radical via one-electron oxidation is possible because of the absence of the N−H bond necessary to produce aminyl radical.We examined the photoinduced reaction product of N,N,N′,N′-tetramethyl-1,4-phenylenediamine (TMPD) and N,N-dimethylaniline (DMA) to confirm the production of the radical species in the CH 2 Br 2 /hexane mixture.−44   hexane do not form TMPD +• or any other product identifiable from the absorption spectra.Even when BQ is used as the electron acceptor in a hexane solution of CsPbBr 3 NCs, TMPD +• was not formed similarly to the case of PTZ (Figure 4e, condition 1).Dication of TMPD, known to form on the electrode via two-electron oxidation, is not observed, likely due to the higher oxidation potential for the second hole transfer. 45hen the photoexcitation is discontinued, the absorption from TMPD +• decayed on the time scale of 10 min.When CdSSe NCs are used instead of CsPbBr 3 NCs, the production of TMPD +• was suppressed under all three solvent conditions (Figure 4e, conditions 2−4), consistent with the observations made for other amines discussed above.
The quantum yield of photogeneration of PTZ +• and TMPD +• is in a 5−10% range under our experimental conditions and comparable to that of the molecular catalyst. 46Calculation in the Supporting Information) Since the interfacial charge transfer to the precursor molecules can be affected by the surface ligands, surface modification that gives the easier access of the precursor to the surface of the NCs should increase the quantum yield. 47While we plan to study the kinetics of radical formation correlated with the NC structure, ligand, and solvent polarity in the future, we confirmed that partially removing the long-chain ligand (oleylammonium bromide) can already increase the quantum yield from ∼5 to >30% for PTZ +• (Figure S8).
Figure 5a shows the time-dependent absorption spectra of the reaction product from a mixture of DMA and CsPbBr 3 NCs in CH 2 Br 2 /hexane under 473 nm excitation.Unlike in the case of TMPD, the absorption spectra in Figure 5a do not correspond to DMA +• .The absence of DMA +• is consistent with its short lifetime (∼μs) reported from the electrochemical oxidation studies of DMA. 48,49As will be discussed further below, the reaction product has been identified as the structure resulting from the dimerization of DMA +• and additional demethylation.No product detectable from UV−visible absorption was formed when only hexane was used as the solvent (Figure 5b) or when CsPbBr 3 NCs were replaced with CdSSe NCs.
Because of the limited structural information that can be obtained from the absorption spectra in Figure 5a, the structure of the product was determined from mass spectrometry, as described in Supporting Information (Experimental Section).In typical electrochemical oxidation of DMA, the observed reaction products are N,N,N′,N′tetramethylbenzidine (TMB) resulting from the dimerization of DMA +• or its single-or two-electron oxidation product (TMB +• or TMB ++ ) depending on the electrode potential. 48In contrast, the photoinduced oxidation of DMA by CsPbBr 3 NCs produces only the demethylation product of TMB (TMB-CH 3 + ), where one of the amine groups transforms into imine.−52 In the mass spectrum of the reaction product shown in Figure 5c, the species at m/z 225.1 correspond to TMB-CH 3 + .In contrast, the peak at m/z 240.1 corresponding to TMB +• that signifies the formation of TMB is absent.The identification of the species at m/z 225.1 as TMB-CH 3 + was made from the tandem mass spectrometry study by fragmenting the species at m/z 225.1 via collision-induced dissociation (CID) into two species whose structures have been determined from the mass spectra.Further discussions on the structural determination are in the Supporting Information (Figure S9).
To further examine if the coupling reaction between two different radical species can be achieved using CsPbBr 3 NCs, we examined the photoinduced reaction product from the mixture of DMA and PTZ in a solution of CsPbBr 3 NCs.The combination of DMA and PTZ is chosen because it has been demonstrated in previous studies that the radicals of DMA and PTZ generated via electrochemical oxidation can undergo a coupling reaction. 42,53The C−N bond formation resulting from the coupling of the two radicals derived from DMA and PTZ also bears a practical importance for the synthesis of pharmaceuticals. 53Figure 6a shows the time-dependent  absorption spectra of the mixture of PTZ, DMA, and CsPbBr 3 NCs in CH 2 Br 2 /hexane under 473 nm excitation, which is the reaction condition that can produce radicals from both PTZ and DMA.Since the identity of the coupling product cannot be determined from the absorption spectra, mass spectrometry was used to determine the structure of the coupling reaction product.The mass spectrum of the reaction products (Figure 6c) indicates that the composition of the products is significantly more complex than that of a single precursor, exhibiting the species derived from each precursor in addition to the coupling product.In the mass spectrum, the species at m/z 319.1 corresponds to the coupling product between the radicals derived from PTZ and DMA, which was previously seen from the electrochemical coupling of DMA +• and PTZ-H • . 42The absence of the peaks at m/z 240.1 (TMB +• ) and m/ z 225.1 (TMB-CH 3 + ) in the mass spectrum indicates that the dimerization of DMA is suppressed when both DMA and PTZ are present in the reactant mixture.Since the reaction was performed in a fully oxidative condition in the CH 2 Br 2 /hexane mixture that limits the generation of PTZ-H • proposed to be involved in the coupling the yield of the product is not optimal for this coupling reaction.Tuning the population of the specific radical species involved in the coupling reaction by varying the solvent composition and altering the degree of redox imbalance will be an interesting direction to explore further.

■ CONCLUSIONS
We report a CsPbBr 3 NC-based photocatalytic approach that can generate stable N-centered organic radicals from amines either oxidatively or via sequential oxidation and reductive N− H bond breaking in a selective manner without the interference from reversible charge transfer.This has been achieved by utilizing the unique capability of CsPbBr 3 NCs to control the population imbalance of electrons and holes via photoinduced halide exchange between the NCs and CH 2 Br 2 that facilitates irreversible electron removal from the photoexcited NCs.The ability to tune charge carrier population imbalance enabled the selective generation of aminium and aminyl radicals from the same secondary aryl amine precursor by varying only the solvent composition, which cannot be readily achieved with other semiconductor NC photocatalysts.In addition, coupling reactions between different N-centered radicals are also demonstrated by using CsPbBr 3 NCs as the effective radical-generating photocatalyst.
■ METHODS Synthesis of OLAB-Passivated CsPbBr 3 and Oleic Acid-Passivated CdSSe NCs.CsPbBr 3 NCs (∼10 nm in size) were synthesized via hot injection following the previously reported method. 54Cs precursor solution (Cs-oleate solution) was prepared by heating the mixture of Cs 2 CO 3 (250 mg), oleic acid (OA, 0.8 g), and 1-octadecene (ODE, 7 g) at 150 °C for >10 min under N 2 flow on a Schlenk line.In a separate flask, Pb/Br precursor solution was prepared by dissolving PbBr 2 (69 mg) in the mixture of oleylamine (OAm, 0.5 mL), OA (0.5 mL), and ODE (5 mL).Pb/Br precursor solution was heated at 120 °C for 10 min under vacuum in a Schlenk line, and the temperature was increased to 200 °C.Subsequently, 0.4 mL of Cs-oleate was injected into this solution to initiate the reaction.After a few seconds of reaction, the reaction was quenched with an ice water bath.The product was recovered by centrifuging the reaction mixture and redispersing the precipitate in hexane.The resulting NCs are Br − -rich on the surface.CdSSe NCs were synthesized employing a previously reported method. 4,55S/Se precursor solution was prepared as S/Se = 10:1 in ODE.A 2 mL portion of the S/Se precursor solution was injected into the mixture of CdO (126 mg), ODE (12 mL), and OA (2.02 g) at 270 °C under N 2 flow to initiate the reaction.After 4 min of reaction at 250 °C, the reaction was quenched by air cooling.The product was recovered by precipitating it with acetone and centrifugation.The precipitate was redispersed in toluene, followed by additional purification, with methanol as the antisolvent.The product is S 2−rich on the surface and passivated with oleic acid.
Synthesis of CsPbBr 3 NCs with a Na + -Rich Surface and CdS NCs with a S 2− -Rich CsPbBr 3 NCs with a Na + -rich surface were prepared by ligand exchange on the OLAB-passivated CsPbBr 3 NCs following the method reported by Dong et al., which forms a stable bipolar shell, with Na + occupying the outermost sites confirmed by the large increase of the positive ζ-potential. 56First, OLABpassivated CsPbBr 3 NCs were precipitated by adding methyl acetate and redispersed in toluene after centrifugation.This procedure was repeated three times to remove OLAB.Subsequently, 10 μL of 1 M phenylethylammonium bromide (PEAB) DMF solution was added and mixed vigorously.PEAB served as an intermediate ligand used before completely replacing the original ligand with NaBr.Finally, 10 μL of saturated NaBr DMF solution was added and mixed vigorously.After removing the excess salt by centrifugation, the product NCs were recovered by precipitation with methyl acetate and redispersed in toluene.No additional organic ligands replacing the OLAB are added, which drastically reduces the facet-to-facet distance of the NCs in the TEM image, as shown in Figure S7.CdS NCs with an S 2− -rich surface were prepared by adding 10 μL of 1 M Na 2 S DMF solution and 4 μL of oleylamine to oleic acid-passivated CdS NCs that are Cd 2+ -rich on the surface.After removing the excess salt by centrifugation, the product NCs were recovered by precipitation with methyl acetate and redispersed in toluene.
Photogeneration of Aminium and Aminyl Radicals by CsPbBr 3 NCs.The stock solutions of PTZ, POZ, TMPD, DMA, and BQ were prepared by dispersing each compound in toluene at 50 mM concentration in a glovebox.All solvents (hexane or CH 2 Br 2 / hexane mixture) used to prepare the reactant mixture were bubbled with N 2 for 30 min to remove the dissolved O 2 .For the photogeneration of the aminium radical, ∼1 mL of the reactant mixture containing CsPbBr 3 NCs (absorbance of 0.1 at the excitation wavelength) and reactant amine (5 mM) dispersed in a CH 2 Br 2 / hexane mixture (1:1 v/v) was prepared in a sealed quartz cuvette.For the photogeneration of the aminyl radical, the reactant mixture was prepared in hexane instead of a CH 2 Br 2 /hexane mixture, while keeping the amount of other components identical.For the coupling reaction of DMA, ∼1 mL of the reactant mixture containing CsPbBr 3 NCs (absorbance of 0.1 at excitation wavelength) and DMA (5 mM) was prepared in a sealed quartz cuvette with CH 2 Br 2 /hexane (volume ratio 1:1) as the solvent.For the coupling reaction of DMA with PTZ, ∼1 mL of the reactant mixture containing CsPbBr 3 NCs (absorbance of 0.1 at excitation wavelength), DMA (5 mM), and PTZ (5 mM) dispersed in the CH 2 Br 2 /hexane solvent (volume ratio 1:1) was prepared in a sealed quartz cuvette.All of the reactant mixtures were purged with N 2 for several minutes to further remove dissolved O 2 before photoexcitation at 473 nm at the intensity of ∼1.7 mW/cm 2 .The concentration of BQ used in the control experiment was 5 mM.The absorbance of CdSSe NCs in the control experiments was also 0.1 at the excitation wavelength.
Optical Measurements and Product Characterization.The absorption spectra of all of the solutions were measured with a CCD spectrometer (Ocean Optics, QE65).EPR spectra were obtained on a Bruker E1 Exsys with a super CW EPR bridge.EPR spectra of the radical products were obtained at room temperature at a 9.37 MHz microwave frequency and 20 mW microwave power.All mass spectrometry (MS) data were acquired on an Orbitrap Velos Pro mass spectrometer (Thermo Fisher Scientific).Raw data were extracted using the Qual Browser of the Xcalibur program (Thermo Fisher Scientific).The following MS parameters were used for all data acquisitions.Samples were analyzed through applications of 1.0−3 kV DC spray voltages.An S-lens RF level was set to be 67.9% for positive ion mode.The capillary temperature was set to be 280 °C.Full MS scans were acquired at a m/z range of 50−500.The resolution was set at 30,000.The microscan number was set at 2, and the maximum injection time was set at 200 ms.Tandem mass spectra were obtained via CID with a normalized collision energy ranging from 20 to 50 arbitrary units.All samples were analyzed via inductive nanoelectrospray ionization, where the electrode does contact the solution being analyzed.The electrode/solution contact was removed to ensure that the reaction of products observed via MS was completed by the photo-oxidation reaction and not from the transfer of electrons to/from the electrode.To fabricate the MS emitters used for analysis, World Precision Instrument borosilicate glass capillaries from Fisher Scientific were used and pulled to a fine tip using a micropipette puller (P-1000, Sutter Instruments).The following parameters were used to pull a capillary with an orifice size of >10 μm: heat = 530, pull = 0, velocity = 22, time = 250, pressure = 250, and ramp = 550.

■ ASSOCIATED CONTENT
* sı Supporting Information The following information is available free of charge: The Supporting Information is available free of charge at https:// pubs.acs.org/doi/10.1021/jacs.3c05323.

Scheme 1 .
Scheme 1.(a) One-Electron Oxidation of Secondary Aryl Amine by CsPbBr 3 NCs in a CH 2 Br 2 /Hexane Mixture Forming Aminium Radical; (b) One-Electron Oxidation and Reductive Deprotonation of Secondary Aryl Amine by CsPbBr 3 NCs in a Hexane Forming Aminyl Radical
Figure 4a,b shows the time-dependent absorption spectra of the reaction products from the mixture of TMPD and CsPbBr 3 NCs in two different solvents under 473 nm excitation.TMPD +• was formed by CsPbBr 3 NCs in the CH 2 Br 2 /hexane mixture via hole transfer without interference from electron transfer.The absorption features in Figure 4a at 575 and 624 nm are attributed to TMPD +• . 35,41In contrast, CsPbBr 3 NCs in

Figure 3 .
Figure3.Time-dependent absorbance of PTZ-H • at 387 nm produced by CsPbBr 3 (green) and CdS (red) NCs with an anion-rich (solid) and cation-rich (dashed) lattice surface.For this comparison, all four NCs were excited at the same excitation rate.The illustration of the NC surface is for the OLAB-passivated CsPbBr 3 NCs with a Br − -rich surface, the CsPbBr 3 NCs with a Na + -rich surface without organic ligands, the oleylaminepassivated CdS NCs with a S 2− -rich surface, and the oleic-acid-passivated CdS NCs with a Cd 2+ -rich surface.

Figure 5 .
Figure 5. (a) Time-dependent absorption spectra of the reaction product of DMA in CH 2 Br 2 /hexane solution of CsPbBr 3 NCs.(b) Absorption spectrum of the product of DMA in hexane solution of CsPbBr 3 NCs or CdSSe QDs in CH 2 Br 2 /hexane after 1 h of reaction.(c) Mass spectrum of the reaction product of DMA in CH 2 Br 2 / hexane solution of CsPbBr 3 NCs.

Table
. Structure of the Precursor Amines and the Radicals Formed by CsPbBr 3 NCs Dispersed in the Given Solvent under 473 nm Excitation weak 473 nm excitation with the corresponding solvent composition.A notable result summarized in Table Dong Hee Son − Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States; Center for Nanomedicine, Institute for Basic Science and Graduate Program of Nano Biomedical Engineering, Yonsei University, Seoul 03722, Republic of Korea; orcid.org/0000-0001-9002-5188; Email: dhson@chem.tamu.edu